1. Field of the Invention
The invention relates generally to a Complementary Metal Oxide Semiconductor (CMOS) image sensor. More particularly, the invention relates to two-dimensional time delay integration visible CMOS image sensor.
2. Description of Related Art
Unmanned Aerial Vehicles (UAVs) are remotely piloted or self-piloted aircrafts that can carry cameras, sensors, and other communication equipment. UAVs may be remotely controlled (e.g. flown by a pilot at a ground control station) or fly autonomously based on pre-programmed flight plans or more complex dynamic automation systems. UAVs are typically used for reconnaissance and intelligence-gathering, and for more challenging roles, including combat missions.
Ideally, an image taken from a camera onboard the UAV should be clear to provide accurate intelligence-gathering and determine appropriate targets. However, since UAVs shake from wind gusts during their flight operation, the image received from UAV is not clear enough to accurately identify targets on the ground. Consequently, there is a low signal to noise ratio due to wind and mechanical vibrations of the camera. This problem is compounded with moving scene imagery.
To improve signal to noise ratio, prior art stabilizers were integrated with the gimbal assembly of high speed cameras onboard the UAVs. The stabilizers reduce interferences caused by wind or mechanical vibrations. Additionally, the signal to noise ratio may be improved using Charge-Coupled Devices (CCDs) with Time Delay Integration (TDI). CCDs with TDI technology allow an image in a charge domain to move at about the same speed as the moving scene or target. However, CCDs with TDI are one dimensional and require multiple chip systems.
Conventional CMOS integrated circuits can achieve TDI in one dimension. The CMOS integrated circuits provide TDI using a switch matrix or a transistor chain CCD equivalent. The switch matrix typically accumulates additional noise and the signal to noise ratio improvement is less than proportional to the square root of the number of TDI channels. The transistor chain CCD equivalent cannot have high QE photodiode and is not a mainstream CMOS or CMOS Image Sensor (CIS) process.
With an ever increasing demand for improved imaging sensors, there remains a need for a two dimensional TDI visible CMOS image sensor that allow a charge to move at the same speed and follow a similar path in the charge domain as the moving image so that more charge from the scene can be integrated resulting in an improved signal to noise ratio. If readout noise is dominant, the signal to noise ratio improvement is proportional to the number of TDI channels.